The present invention relates generally to electric motors, and more specifically, but not exclusively, to estimating a critical temperature of a vector-controlled AC induction motor.
In a traction drive for an electric vehicle (EV), a load and a speed of the drive is user dependent. This is in contrast to other uses of traction drives where the load and speed are set by the type of application and installation details. EV traction drives have high peak to continuous power ratios, and include temperatures for various components that can vary widely. To prevent damage to a component of the traction drive due to temperature, sometimes a motor controller regulates output power within the limits of a thermally limiting component.
Depending upon many factors including design and implementation, for many motors it is possible to obtain desirable performance levels through simple-to-achieve thermal measure of a motor component (e.g., stator windings of the motor). The motor controller uses this direct thermal measurement to regulate output power and protect the entire motor from damage due to excess temperature.
However, depending on electromagnetic design, geometry, and cooling system choices, such a measurement may not indicate the thermally limiting component in all circumstances. Characterization of the electric motor may indicate that maintaining the temperature of the stator windings within the thermal limits of stator winding insulation may not maintain other components within their appropriate thermal limits (e.g., components thermally coupled to a rotor of the electric motor).
For instance, while the components of an AC induction squirrel cage rotor can operate at high temperatures (i.e., shaft, magnetic steel, bars/aluminum), there are bearings that are thermally well-coupled to the rotor shaft and these bearings typically have a much lower operating temperature limit than the other components. In this AC induction motor, it is extremely hard to measure directly the temperature of the rotor or the bearings. Additionally, many methods of directed rotor and bearing cooling come with efficiency penalties or mechanical design challenges. Thus, in such a system where a primary thermal rejection path for the rotor is via convection across an air gap to the stator, it is critical to have some quantification of the temperature of the thermally limiting component (e.g., the bearing(s)).
Without direct measurement of the bearing temperature, the motor controller could use a direct thermal measurement of another component of the motor and limit output power according to some relationship between the temperature of the measurable component and the temperature of the thermally limiting component. Sometimes this is referred to as a proxy, as in the directly measurable component serves as a proxy for the component which is not measurable or measured. The more thermally remote and inaccurate the proxy relationship, the more inefficient the motor controller will be in regulating the output power. Often this means the performance of the motor is not optimal as its performance is limited more than necessary to allow for a sufficient safety margin.
To improve this situation, and allow more optimization in electric motor performance and design, one either must find a better proxy (which may not be available), directly measure the critical component (which can introduce different complications and undesirable inefficiencies), and/or estimate the temperature of the critical component, which in the case of the bearings would extend their serviceable life.
In many conventional systems, a motor's continuous power is characterized based on information available from direct thermal measurements only. Doing so, especially in designs with no direct rotor cooling and widely varying inlet air temperature (results in widely varying stator heat rejection), results in poor reliability or performance as a compromise between worst and best case delta temperature between directly measured thermals and limiting, non-measured, components is achieved. To reduce the need for such compromises, what is needed is a system and method including a runtime thermal model that estimates individual component temperatures, permitting the system to only limit power when necessary or desirable.